1. Field of the Invention
This invention relates to an sighting apparatus using a laser beam, and more particularly to such apparatus which is capable of emitting a laser beam in any horizontal direction by remote control.
2. Description of the Prior Art
A sighting apparatus of this kind is already disclosed in the Japanese Patent Application Laid-Open Nos. 215913/1986 and 215914/1986. This apparatus includes a rotatable table supported on a base, a laser oscillator and an optical system which guides the laser beams emitted from the laser oscillator in the horizontal direction whereby the table is rotated by remote control. Even a sole user can perform a survey using this apparatus, so that the use of this apparatus has spread.
As shown in FIG. 5, the optical system includes a collection lens 3 which collects the laser beams 2 emitted from the laser oscillator 1, an automatic correction mechanism 4 which corrects the laser beams passing through the collection lens 3 so as to emit horizontally, an objective 5 which images the laser beams from correction mechanism 4 onto an object as a reference point or line, and a focusing lens 6 which is disposed movably in the direction of the optical axis between the correction mechanism 4 and the objective 5. The automatic correction mechanism 4 has a function of guiding the laser beams horizontally at all times and includes three mirrors which comprise two normally fixed tilt mirrors 4a and 4b and a single flat plate-like mirror 4c suspended by a thread.
In the conventional sighting apparatus, a movable mirror cylinder which holds the focusing lens 6 is usually mounted slidably within a fixed mirror cylinder which holds the objective 5. The movable mirror cylinder is manually moved relying upon scales marked between both the mirror cylinders to perform focusing. Therefore, there is the problem that if the distance determination is performed erroneously, the area of the reference point or line focused onto the object would be increased to thereby render it difficult to find a correct reference. In order to solve this problem, the user must return to the place where the sighting apparatus is installed to troublesomely rearrange the movable mirror cylinder. Thus the solution of this problem is desired.